Help me in solving TILINGS problem

help
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1

My issue

f(n)=f(n-1)+f(n-2)+2f(n-3)
How above recurrence relation derived for finding number of ways to tiling of 2N size room.
Problem Link: CodeChef: Practical coding for everyone

2

@nadiyasoaib
U can get it by observation on smaller cases like n=1,2,3,4 and so on …